The present invention relates to a spinel-type lithium-manganese oxide containing at least one heteroelement (M) other than Li and Mn, as well as a preparation process and the use thereof.
Manganese oxides have been used for many years as an active material in electric cells. In recent years, lithium-manganese oxides which are composite materials of manganese and lithium as well as lithium-manganese oxides in which manganese in said lithium-manganese oxides is partially replaced by at least one heteroelement have attracted attention for use as an active material for positive electrodes of lithium secondary batteries which are capable of providing high output and high energy density.
Composite oxides of Li and various metals such as Co, Ni, Mn have been proposed as a material for positive electrode of lithium secondary batteries, which are required to have a high voltage working range, a high discharge capacity and a high cycle stability of charge and discharge.
A spinel-type LiMn2O4, which is one type of a composite oxide of Li and Mn, has been known to show a two-stage discharge, the first discharge stage being at a level of near 4V and the second discharge stage being at a level of near 3V. It seems to be promising as an active material for a positive electrode because it would be expected to provide high energy output if it could be reversibly cycled in a working range around 4V.
However, it has recently been found that Mn in the LiMn2O4 structure dissolves in organic electrolytes when charge and discharge is conducted using LiMn2O4 as an active material for lithium secondary batteries. Furthermore, our experiments revealed that as much as 1 mol % of the Mn content in the structure may dissolve when LiMn2O4 is merely stored at 85xc2x0 C. in some organic electrolytes without performing charge and discharge, and that characteristics as an active material for a positive electrode significantly deteriorate after dissolution.
This means that Mn in the LiMn2O4 structure used as a positive electrode for lithium secondary batteries may dissolve in organic electrolytes after long-term storage without performing charge and discharge, thereby causing a failure of the positive electrode in lithium secondary batteries.
It is an object of the present invention to provide a high-performance spinel-type lithium-manganese oxide for use as a material for positive electrodes of a Li secondary battery with inhibited Mn dissolution in an organic electrolyte, as well as a high-performance lithium secondary battery using said lithium-manganese oxide as a positive electrode.
As a result of careful investigation, it has been found that the above object can be achieved by using a spinel-type lithium-manganese oxide containing at least one heteroelement (M) other than Li and Mn wherein Mn is replaced by Li and M, represented by the following formula:
{Li}[Lix.My.Mn(2xe2x88x92xxe2x88x92y)]O4+d
wherein { } represents the oxygen tetrahedral sites (8a sites) in the spinel structure and [ ] represents the oxygen octahedral sites (16d sites) in the spinel structure, 0 less than xxe2x89xa60.33, 0 less than yxe2x89xa61.0, xe2x88x920.5 less than d less than 0.8, with said d value being negative when the calcination atmosphere is a reducing atmosphere, and being positive when it is an oxidizing atmosphere, and M represents at least one heteroelement other than Li and Mn.
Further, we found a process for preparing the spinel-type lithium-manganese oxide containing at least one heteroelement (M) other than Li and Mn according to the present invention as well as a high-performance lithium secondary battery using the spinel-type lithium-manganese oxide containing at least one heteroelement (M) other than Li and Mn according to the present invention as an active material for a positive electrode, and thus accomplished the present invention.